Matrix wire print head with free bending print wires

ABSTRACT

This invention relates to an improved wire matrix print head having significantly reduced friction on the print wires and to a method for the manufacture of the improved print head. The print head has an element formed at one end for mounting the nonprinting end of the wires and a print guide member having end openings arranged in a predetermined print pattern mounted at the other end of the print head, the print wires each passing through and being supported in a separate one of the openings. First and second guide plates are also provided, each of the plates having n openings formed therethrough which openings are arranged in predetermined patterns and through which openings the print wires are adapted to pass. Each wire passes through a separate opening in each of the guide plates. The guide plates are mounted at predetermined positions between the nonprinting end of the wires and the print guide member. The predetermined positions are selected such that the first guide plate is spaced from the print guide member by a first predetermined distance, the guide plates are spaced from each other by a second predetermined distance, and the second guide plate is spaced from the nonprinting ends of the wires by a third predetermined distance, the first, second and third predetermined distances all being shorter than the critical length for the print wire to prevent buckling. The predetermined positions at which the guide plates are mounted are also both within one-eighth wave-length of the peak deflection points of a print wire when the wire is vibrating at its natural frequency of vibration. For a preferred embodiment of the invention, the print head is of the type having a single column of n print wires and the print guide member has its n openings alined one on top of the other, there being a predetermined spacing between adjacent openings. The nonprinting ends of the wires are mounted with one wire in the middle and the other (n-1) wires substantially symmetrically positioned around the one wire with the angular spacing between adjacent wires being substantially equal. The guide plates each have a center opening through which one wire passes, and (n-1) other openings positioned around the center opening through a separate one of which each of the other (n-1) wires passes. The spacing of each of the openings from the corresponding center opening is a function of the natural deflection of the print wire passing therethrough at the predetermined position of the guide plate.

States Kwan et a1.

atent [1 1 Sept. 23, 1975 MATRIX WIRE PRINT HEAD WITH FREE BENDING PRINT WIRES [73] Assignee: The Bunker Ramo Corporation, Oak

Brook, [11.

[22] Filed: July 12, 1973 [21] Appl. No.: 378,774

Kodis 197/1' R Primary Examiner-Edgar Burr Assistant ExaminerR. T. Rader Attorney, Agent, or Firm-Frederick M. Arbuckle [5 7] ABSTRACT This invention relates to an improved wire matrix print head having significantly reduced friction on the print wires and to a method for the manufacture of the improved print head. The print head has an element formed at one end for mounting the nonprinting end of the wires and a print guide member having end openings arranged in a predetermined print pattern mounted at the other end of the print head, the print wires each passing through and being supported in a separate one of the openings. First and second guide plates are also provided, each of the plates having n openings formed therethrough which openings are arranged in predetermined patterns and through which openings the print wires are adapted to pass. Each wire passes through a separate opening in each of the guide plates. The guide plates are mounted at predetermined positions between the nonprinting end of the wires and the print guide member. The predetermined positions are selected such that the first guide plate is spaced from the print guide member by a first predetermined distance, the guide plates are spaced from, each other by a second predetermined distance, and the second guide plate is spaced from the nonprinting ends of the wires by a third predetermined distance, the first, second and third predetermined distances all being shorter than the critical length for the print wire to prevent buckling. The predetermined positions at which the guide plates are mounted are also both within one-eighth wave-length of the peak deflection points of a print wire when the wire is vibrating at its natural frequency of vibration. For a preferred embodiment of the invention, the print head is of the type having a single column of n print wires and the print guide member has its n openings alined one on top of the other, there being a predetermined spacing between adjacent openings. The nonprinting ends of the wires are mounted with one wire in the middle and the other (n-l wires substantially symmetrically positioned around the one wire with the angular spacing between adjacent wires being substantially equal. The guide plates each have a center opening through which one wire passes, and (n-1) other openings positioned around the center opening through a separate one of which each of the other (nl) wires passes. The spacing of each of the openings from the corresponding center opening is a function of the natural deflection of the print wire passing therethrough at the predetermined position of the guide plate.

8 Claims, 9 Drawing Figures US Patent Sept. 23,1975 Sheet 1 Of4 3,907,092

In J 5 H 4 US Paten lllmll W IIHI Shee 3 of 4 I NW! III" US Patent Sept. 23,1975

Sheet 4 of 4 3,907,092

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MATRIX WIRE PRINT HEAD WITH FREE BENDING PRINT WIRES This invention relates to an improved wire matrix print head having significantly reduced friction on the print wires and to a method for the manufacture of the improved print head.

BACKGROUND OF THE INVENTION Wire matrix print heads, because of their light weight and short print stroke, are used extensively in high speed electronic printers. Some of these print heads contain enough print wires to print a whole character (for example, 35 wires arranged in a 5 X7 matrix). However, lighter weight and simpler heads containing only a single column of print wires have been found more suitable for high speed printing, particularly when printing on the fly. With the latter tyoe of head, the head is energized once for each stroke of the character. Thus, for a character formed from a 5 7matrix of dots, the head would contain seven print wires and would be selectively actuated five times for each character. A print head of this type is disclosed in some detail in the US. Pat. No. 3,690,431, entitled PrintheadAssembly Containing Solenoids, issued Sept. 12, 1972 to Robert Howard. 1

Matrix wire print heads of the type indicated above normally employ a separate solenoid for advancing each print wire to a print position with a separate-spring being provided to return the wire to its initial position. Since the spacing between the wires at the print end of the print head is very small, normallybeing in the order of a few thousandths of an inch, the wires mustbe bent in the print head to provide adequate room at the rear of the head for mounting the drive solenoids, return springs, and other print control elements. Guide elements must therefore also be provided in the print head to lead the wires from the solenoids to the print position at the front of the head.

In addition, the wires, being relatively thin, tend to buckle and vibrate when they impact a document during a print operation. In order to prevent buckling and vibration, print heads, such as that described in the above-mentioned Howard patentQprovi'de guide tubes along most of the length of the wire through which the wire is passed. I

However, the use of guide tubes to prevent buckling and vibration of the print wires presents'serious design and operational problems in the print head. First, the tubes must be carefully designed to conform with the bent shape of the wire so as to prevent the wire from grabbing or being held up in the'tubef Even with careful design, there is substantial friction between the wires and the tubes which significantly increases theenergy required both of the solenoid to drive the wire and of the return spring to bring the wire back to its initial position. This means that larger, more expensive solenoids must be utilized and stronger springs must be'utilized to return the wires, and therefore more' current must be utilized to drive the solenoids. The use of the guide tubes creates critical alinement'problemswhich, in addition to the wear on the wires and'gui'des resulting from friction, results in increased maintenance requirements on the head and significantlyreduces the life of the heads and of the various elements thereof. Since the heads are designed to operate'at speeds of up to 200 characters per second, the heads are being'open ated at speeds of up to 1,200 operations per second. Friction between the print wire and guide tubes or plates thus also results in significant heat which causes further damage to the components of the head and creates dissipation problems. The problems indicated above also serve to limit the speed at which the head may be operated without serious damage.

It is therefore apparent that a need exists for an improved wire matrix print head which is designed so as not to require guide tubes in order to prevent buckling and vibration of the print wires, and which is otherwise designed to minimize friction between the print wires and the elements utilized to support and guide the wires. Such a technique should also reduce the number of components required in the head and the tolerances in the manufacture of the head so as to reduce the cost of fabricatingthe head, reduce the maintenance required on the head, and. increase the expected head life. l

SUMMARY OF THE INVENTION In accordance with the above, this invention provides a print head of the type having a matrix of n print wires. There is a means formed at one end of the head for mounting the 'non'printing end of the wires and a print guide member having end openings arranged in a predetermined print pattern mounted at the other end of the print head, the print wires each passing through and being supported in a separate one of the openings. First and second guide plates are also provided, each of the plates having n openings formed therethrough which openings are arranged in predetermined patterns and through which openings the print wires are adapted to pass. Each wire passes through a separate opening in each of the guide plates. The head has a means for mounting the guide plates at predetermined positions between the means for mounting the nonprinting end of the wires and the print guide member. The predetermined positions are selected such that the first guide plate is spaced from the print guide member by a first predetermined distance, the guide plates are spaced from each other by a second predetermined distance, and the second guide plate is spaced from the means for supporting the nonprinting ends of the wires by a third predetermined distance, the first, second and third predetermined distances all being shorter than the critical length for the print wire to prevent buckling. The predetermined positions :at which the guide plates are mounted are also both within one-eighth wavelength of the peak deflection points of a print wire when the wire is vibrating at its natural frequency of vi bration.

For a preferred embodiment of the invention, the print head is of the type having a single column of n print wires and the print guide member has its n openings aligned one on top of the other, there being a predetermined spacing between adjacent openings. The means for mounting the nonprinting ends of the wires mounts the wires with one wire in the middle and the other (n 1) wires substantially symmetrically positioned around the one wire with the angular spacing between adjacent wires being substantially equal. The guide plates each have a center opening through which one wire passes, and (11-1) other openings positioned around the center opening through a separate one of which each of the other (n-I) wires passes. The spacing of each of the openings from the corresponding center opening is a function of the natural deflection of the print wire passing therethrough at the predetermined position of the guide plate.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially cut-awaytop view of a print head of a preferred'embodiment of the invention.

FIG. 2 is a sectional side view of the print head taken along the line 22 of FIG. 1.

FIG. 3 is a back view of the print head shown in FIG.

FIG. 4 is a partial sectional view along the line 44 of FIG. 2 showing the back guide plate and also illustrating some of the parameters utililzed in determining the positions of the openings in this plate.

FIG. 5 is a partial sectional view along the line 5-5 of FIG. 2 showing the front guide plate and some of the parameters utilized in determining the positions of the openings in this plate.

FIG. 6 is a partial front view taken along the line 66 ofFIG. 2 showing the print guide member.

FIG. 7 is a diagram of a print wire illustrating some of the parameters utilized in making the various'positioning determinations of this invention.

FIG. 8 is an enlarged view of the left portion of FIG. 7, illustrating additional parameters utilized in making the positioning determinations.

FIG..9 is a diagram of a print wire illustrating its vibration modes'and some-additional positioning parameters.

I DETAILED DESCRIPTION Referring now to the figures, and in particular to FIGS. 1 and 2, it is seen that the print head 10 of this invention has a metallic housing1l2 with a raised rear wall 14. Seven solenoid assemblies 16v are mounted in corresponding openings 17 in the rear wall 14. A print wire 18 projects from each solenoid assembly 16. Each of the solenoid assemblies 16 may be considered to be of the type shown in the beforementioned Howard patent with the wire 18 terminating in and being attached to the armature of a solenoid (not shown) and the armature terminating in and being attached to the hub of a wagon wheel or other shaped spring. When the armature is energized, the print wire moves forward by a distance of roughly twenty-thousandths of an inch, the spring returning the wire to its initial position when current is removed from the solenoids. The exact nature of the solenoid assemblies 16 does not form part of the present invention and will not therefore be described further herein.

Mounted in the front of head 10 is a jewel 20 having seven alined openings 22 formed therein. A corresponding print wire 18 is supported in each of the openings 22 with the end of the wire normally being flush with the front surface of the jewel. The center-tocenter spacing of the openings 22 is quite small, being 14 mils (14-thousandths of an inch) for an exemplary embodiment of the invention. The size of the openings 22 is slightly larger than the diameter of the wires 18.

Housing 12 also has a first pair of guide slots or brackets 24 and a second pair of guide slots or brackets 26 formed therein. A first guide plate 28 having seven openings 30 formed therethrough (FIG. 5) is mounted in brackets 24 and a second guide plate 32 having seven openings 34 (FIG. 4) formed therethrough is mounted in brackets 26. Each of the openings 30 has a diameter which is slightly larger than the diameter of the print wire 18 while each of the openings 34 has a somewhat larger diameter. For example, with a wire di ameter of 14 mils, the diameter of the openings 30 might be 16 mils and the diameter of the openings 34, 22 mils. The locations of the plates 28 and 32 in housing 12 and the location of the openings 30 and 34 in their respective plates are critical to the improved performance of the print head and the manner in which these positions are selected will be described shortly.

The remaining elements of print head 10, which elements do not form part of the present invention, include a pair of mounting flanges 36 formed on housing 12 and a cover plate 38. Other optional elements which do not form part of the present invention include jewels 40 (only jewel 40A is shown in FIG. 5) and 42 (only jewel 42A is shown in FIGS. 1, 2, and 4) which may be mounted in openings 30 and 34 respectively to further reduce friction between the wires and the guide plates. Since, as will be seen later, the print wires do not normally contact the walls of openings 34, a jewel would not normally be used inthese openings except for print head design to operate at extremely high speed. To further reduce friction between the print wires and the guide plates, a coating 44 of a synthetic grease of high viscosity (FIGS. 1 and 2) may be put on either one or both sides of the plates 28 and 32 and over the portion of the print wires 18 passing therethrough. The grease lubrication may be used instead of or in addition to the jewels 40 and 42. Another optional element is a guide tube 45 extending for a short distance from each opening 17 through which tube the corresponding wire 18 passes.

DETERMINATION OF ELEMENT POSITION AND ANGLES In order to eliminate buckling and vibration in the print wires without requiring guide tubes, and to minimize friction between the print wires and the guide plates 28 and 32, the positions of the guide plates in housing 12, the positions of the openings 30 and 34 in the guide plates, and the positions and angles for the solenoid assemblies 16 must all be carefully selected. Some things are initially decided upon such as the diameter of the wires 18, the material which is to be utilized for the wires, the center-to-center spacing of the openings 22 in jewel 20 (this normally being substantially equal to the center-to-center spacing between ad.- jacent wires), and the distance between back wall 14 of housing 12 and jewel 20. From the wire material and dimensions, various constants of the wire such as the Young Modulus of Elasticity (E) and the moment of inertia (I) of the wire may be determined. The maximum force which will be applied to the end of a print wire when it impacts :1 document during a print operation is also either known or easily determined. This gives the critical buckling load (P) on the wire.

Knowing the critical buckling load (P), the Young Modulus of Elasticity (E), and the moment of inertia (I), the critical length (L) to prevent buckling of straight print wires can be computed from the following equation (I):

Thus, to prevent buckling of the print wires under a load force of P or less, the length of wire between jewel 20 and guide plates 28 (L1 in FIG. 7), the length of wire between guide plates 28 and 32 (L2), and the length of wire between guide plate 32 and rear wall 14 (or the end of tube 45) (L3) must all be less than the critical length (L). This then is one criteria for selecting the positions of plates 28 and 32.

It is noted that if the length of housing is too great to permit all three sections to be within the required length, tubes 45 may be utilized to shorten the effective length of the wire. In the discussion to follow, the length of the wire L, will be the effective length of the wire, the length of the wire projecting from tube 45.

In order to eliminate, or at least damp, vibration in wires 18, plates 28 and 32 should also be placed at, or within at least a one-eighth wavelength of, the peak vibration points of the print wire when it is vibrating at its fundamental frequency. Since the print wires are restrained at their ends (or effective end), a print wire vibrating at its fundamental natural frequency would, at the peak points of its vibration, assume one of the two configurations shown in FIG. 9 (one configuration being shown solid and the other dotted). The stiffness of the wire is such that all but the fundamental vibrating frequency may be ignored. Thus, forward guide plate 28 should be positioned within the quarter wavelength section defined by darkened line 50 in FIG. 9, and rear guide plate 32 should be positioned within the quarter wavelength section defined by darkened line portion 52. In each instance, the plate should be placed as close to the peak vibration point as possible.

Unfortunately, the number of parameters involved determining the sections 50 and 52 is so great as to make a mathematical determination of the sections difficult. Best results have therefore been achieved by initially positioning the guide plates by approximation and then determining the exact positions for these plates empirically in accordance with the criteria indicated above. For an exemplary embodiment of the invention, with a wire .14 mills in diameter, Ll was found to be roughly 0.4L, and L,, the distance between the front of the head and plate 32, was found to be roughly 0.7L where L, is the effective length of the print wire.

Once the positions of plates 28 and 32 have been determined, these values may be utilized to determine the positions and angles for solenoid assemblies 16, and the positions of the openings 30 and 34 in plates 28 and 32 respectively. From the figures it is seen that one of the solenoid assemblies 16A is mounted parallel to the axis of housing 12 in an opening 17A formed near the center of rear wall 14. Print wire 18A projects undeflected through a center opening 34A in plate 32, a center opening 30A in plate 28, and a center opening 22A in jewel 20. The other six solenoid assemblies and wires are grouped around the center solenoid assembly and wire in a predetermined, substantially symmetrical pattern. In determining the positions of the solenoid assemblies and openings in this pattern, a starting point is the position of the opening 22 in jewel through which a given wire passes relative to the position of opening 22A. Thus, the starting point for determining the position of each of the openings 30 is displaced from opening 30A by the same distance as the corresponding opening 22 is displaced from opening 22A (FIG. 5). The starting points for determining the positions of openings 34 are similarly displaced as is shown in FIG. 4, as are the starting points for determining the positions of the openings 17 in wall 14. Stated another way, the starting points for computations are the points which the wires 18 would pass through in the plates or wall if the wire projected back from jewel 20 undeflected and parallel to wire 18A. This line, which passes through each of the starting points for the given wire 18, is the line 54 shown in FIG. 7.

To determine the position of the opening 17 for mounting each solenoid assemblies 16, a displacement S of the solenoid from the starting point defined by line 54 is first selected. Solenoid assembly 168 is then positioned a distance S from the starting point for wire 18B and directly above the starting point. The remaining solenoid assemblies are then positioned, as shown in FIG. 3, with each solenoid assembly a distance S from its corresponding starting point and angularly displaced from the adjacent solenoid assembly by (for a seven wire configuration, i.e. N 7). The 60 angular spacing assures equal angular spacing between the solenoid assemblies In making remaining calculations for the placement of the solenoid assemblies 16' and openings 30 and 34, an X-axis 56 will be assumed which is a straight line between the opening 17 in wall 14 in which the solenoid assembly 16 is mounted or the end of tube 45 if tubes 45 are employed and the opening 22 in jewel 20 where the wire terminates. X-axis 56 represents the undefiected condition of the wire 18. A Y-axis is defined which is perpendicular to the X-axis 56. What is desired is to orient the wire such that it will be naturally deflected as if a force W was applied at a point B along the wire, which point corresponds to the position of guide plate 28. This natural deflection of the wire is such that at a point A (the forward or print end of wire 18) the wire is parallel to print head axis line 54, no

' force being required to be applied, or being applied at point C where plate 32 is located. For purposes of mak' ing the required calculations, the wire may be considered to be a beam supported at its ends, and known equations for such structures utilized. Thus, the re- For equation (2), E and I have previously been defined and are constants and S and L, have previously been selected.

. L. Sln L,

gives the angle [3 which is the angle between line 54 and X-axis 56 or, in other words, the angle between wire 18 and the X-axis at point A. The distance (I along the X- axis may be assumed to be the same as the distance Ll for purposes of doing the following calculations and L1 may later be adjusted or a adjusted and the calculations iteratively repeated to compensate for any errors which this assumption introduces. As indicated above,

(sin provides the angle [3. The angle 6 between the wire 18 and the X-axis at point D where solenoid assembly 16 is mounted or tube 45 terminates may be derived from the equation Having determined tan 6, 0 may be easily computed. In some instances, the computations may be simplified by substituting the value for W in equation (2) into equation (3).

The angle alpha (a) at which each solenoid assembly 16 is mounted to the horizontal axis 54 is For X a, this equation reduces to Again the value for W in equation (2) may be utilized, if desired, to simplify equation 6.

Referring now to FIG. 8, it is seen that with the angle B known, the length a known, and the deflection yl known, r1 may be determined by either using the values a and y] to determine the length q for triangle 1, using standard geometry equations, and then use the known values q and L1 to compute r1 for triangle II, or by using the length a and the angle 3 to compute the length of side I, determine the length of side it by subtracting yl from t, and then use the value it and the angle B in triangle III to determine the length rl.

Once fl is determined, the exact position of each opening 30 is determined, as may be seen in FIG. 5, by placing opening 308 a distance rl from the starting point for the B wire directly in line with and above the WuC WIN b hc-] Again, for purposes of the calculations, the displacement for openings 34 (FIG. 4) may be determined from deflection \'2 utilizing standard geometry equations in the same way that rl was determined from y 1. Once r2 is determined, it is utilized, as shown in FIG. 4, to determine the exact position for each of the openings 34 in the same manner previously indicated for determining the positions for the openings 30.

It is noted that the deflection yl and y2 are the natural deflections which occur when the wire is mounted and held at one end at the angle a and is mounted and held at the other end (the A end) parallel to the axis 54. Thus, plate 32 is not required to apply any force to the print wires to cause deflection thereof. The openings 34, being slightly larger than the print wires, would thus not normally contact the wires except on impact to prevent buckling of the wires and to damp vibration. Since buckling primarily occurs in the forward section of the wire, plate 32 is included primarily for damping vibration and the openings therein are large enough (being as indicated previously, several mills in diameter larger than the openings 30 in plate 28) so that there is little if any contact between the wires and the plate. Thus, by the judicious selection of solenoid assembly angle and position, of guide plate positions, and of guide plate opening positions, a print head has been provided which eliminates print wire buckling and vibration without requiring guide tubes and which provides very little friction between the print wires and the guide plates. This virtual elimination of friction in the head permits significantly less current to be used, permits smaller and less expensive solenoids to be used in assemblies l6, permits smaller return springs to be used in the assemblies, and permits for more rapid operation of the print head without generating excessive heat or wear on the components thereof. A head which is thus significantly simpler and less expensive to both build and maintain and which has a significantly longer operation life is provided.

While the symmetrical positioning of the print elements to substantially eliminate friction between the guide plates and the print wires would be significantly more difficult in a character matrix print head having, for example, 35 print wires, the elimination of the requirement for guide tubes or similar members by the judicious positioning of the guide plates to eliminate buckling and vibration should be applicable in any matrix wire print head. Thus, while the invention has been particularly shown and described above with reference to a preferred embodiment thereof, it will be apparent to those skilled in the art that the foregoing and other changes in form and detail may be made therein while still remaining within the spirit and scope of the invention.

What is claimed is:

1. A wire matrix print head employing a plurality of print wires on which friction is reduced, although all or all but one of said print wires are bent between the printing and non-printing ends thereof, the use of guide tubes therebetween being eliminated, comprising a plurality of n print wires each having a printing and a nonprinting end thereon, said It print wires each having a positionally determined natural deflection with peak vibrating deflection points when vibrating at its natural frequency of vibration and a critical length L for a straight unsupported section thereof defined by where P critical buckling load E Youngs Modulus of Elasticity l moment of inertia,

b. means for mounting the non-printing ends of said print wires with one wire in the middle and the other (Ii-l wires substantially symmetrically positioned around said one wire with the angular spacing between adjacent wires being substantially equal,

c. a print guide member having a openings therein with a predetermined spacing between adjacent openings for supporting one of said print wires on said print end thereof which passes through each of said openings,

d. first and second guide plates,

e. means for mounting said first and second guide plates at predetermined positions between said means for mounting the non-printing ends of said wires and said print guide member, said predetermined positions being such that said first guide plate is spaced from said print guide member by a first predetermined distance, said first and second guide plates being spaced from each other by a second predetermined distance, and said second guide plate being spaced from said means for supporting the non-printing ends of the wires by a third predetermined distance, said first, second and third predetermined distances all being shorter than said critical length for each of said unsupported sections of said print wires to prevent the buckling of said sections of said print wires,

f. said first and second guide plates each having a center opening through which said one wire passes, and (-l) other openings positioned around the center opening through a separate one of which each of the other (nl) wires passes, the spacing of each of said openings from the corresponding center opening being a function of said natural deand wherein the natural deflection of the print wire at the position of the second guide plate is WuC from which the positions of the holes in the guide plates are determined.

4. A print head as claimed in claim 1 including a jewel mounted in each of the openings in at least one of said guide plates to reduce the friction on a wire passing therethrough.

5. A print head as claimed in claim 1 including a synthetic grease lubricant coated on at least one side of at least one of said guide plates and over the portions of the print wires adjacent thereto to reduce the friction between the guide plates and the wires.

6. A print head as claimed in claim 1 wherein the openings in said second guide: plate are larger than the openings in said first guide plate, said openings in the second guide plate being large enough so that the wires passing therethrough do not normally Contact the walls thereof.

7. A print head as claimed in claim 1 wherein there are seven print wires in said head; and

wherein the angular spacing between adjacent wires is roughly 60.

8. A print head as claimed. in claim 1 wherein said spacing for each of said (n-l other openings in each of said first and second guide plates is determined by positioning said other openings in an equal angular distribution around the corresponding center openings in said guide plates by a distance corresponding to the point of intersection of said guide plates and said natural deflection of said print wires. 

1. A wire matrix print head employing a plurality of print wires on which friction is reduced, although all or all but one of said print wires are bent between the printing and non-printing ends thereof, the use of guide tubes therebetween being eliminated, comprising a plurality of n print wires each having a printing and a nonprinting end thereon, said n print wires each having a positionally determined natural deflection with peak vibrating deflection points when vibrating at its natural frequency of vibration and a critical length L for a straight unsupported section thereof defined by
 2. A print head as claimed in claim 1 wherein said predetermined positions at which said first and second guide plates are mounted are both within one-eighth wavelength said peak vibrating deflection points of a print wire when the wire is vibrating at its natural frequency of vibration.
 3. A print head as claimed in claim 1 wherein the natural deflection of the print wire at the position of the first guide plate is
 4. A print head as claimed in claim 1 including a jewel mounted in each of the openings in at least one of said guide plates to reduce the friction on a wire passing therethrough.
 5. A print head as claimed in claim 1 including a synthetic grease lubricant coated on at least one side of at least one of said guide plates and over the portions of the print wires adjacent thereto to reduce the friction between the guide plates and the wires.
 6. A print head as claimed in claim 1 wherein the openings in said second guide plate are larger than the openings in said first guide plate, said openings in the second guide plate being large enough so that the wires passing therethrough do not normally contact the walls thereof.
 7. A print head as claimed in claim 1 wherein there are seven print wires in said head; and wherein the angular spacing between adjacent wires is roughly 60*.
 8. A print head as claimed in claim 1 wherein said spacing for each of said (n-1) other openings in each of said first and second guide plates is determined by positioning said other openings in an equal angular distribution around the corresponding center openings in said guide plates by a distance corresponding to the point of intersection of said guide plates and said natural deflection of said print wires. 